The present invention relates to fixation of intervertebral implants and, in particular, it concerns intervertebral disc replacements or spacers which have stepped endplate surfaces to provide stable load support via cortical and endplate tissues combined with prompt fixation to promote the desired osteointegration.
Intervertebral implants such as total disc replacements (“TDR”) and intervertebral spacers (or “cages”) must be firmly fixed to the vertebral endplates immediately after implantation. Post-operative migration of intervertebral implants is a serious complication. Such migration may occur into cancellous bone core of the vertebral body (“subsidence”), and/or to any direction outside to the intervertebral space, damaging the vertebral body and disrupting the vertebral alignment. Implant migration also carries with it a high risk of both functional failure and damage to the nervous and/or visceral structures. Migration of an artificial disc may require either removal of the disc or disc maintenance, both followed by fusion, or re-implantation or repositioning of a TDR. Other risks include facet joints disruption. Thus, permanent prompt stable fixation of the implant is crucial in order for implants to withstand the compressive, shear and torque forces occurring within the intervertebral space.
Various attempts have been made to develop intervertebral implants which achieve both immediate fixation and long-term post-implantation stability. Approaches commonly used in existing intervertebral implants typically try to achieve effective fixation by increasing the contact area or “footprint” of the device, and by employing auxiliary fixation elements such as pins, nails, or screws, or by addition of vertical ribs or keels on the contact surfaces themselves. Despite expectations that the penetration of these auxiliary elements into the vertebral body should provide firm fixation, the results have not proven to be consistent. Furthermore, the perforation of sharp anchoring elements into the vertebral body carries with it the risk of vertebral body fracture and/or failure to osteointegrate.
A further problem hampering stable intervertebral implant fixation is the need to restore the natural arch of the spinal column known as lordosis. In order to reproduce this natural curvature, an intervertebral implant should provide an angle of inclination between the upper and lower fixation surfaces. However, such an angle of inclination generates a wedge-like overall profile which, when combined with normal axial loading of the vertebral column, may contribute to migration of the implant.
Some examples of intervertebral implants illustrating various approaches to fixation may be found in the following US patents and published applications nos.: U.S. Pat. Nos. 5,425,773; 6,989,032; 6,994,727; 7,048,766; 2004/0073311; and 2004/0225364.
There is therefore a need for an intervertebral implant with a tiered endplate surface to provide stable load support via cortical tissue combined with a desired degree of lordosis. It would also be advantageous to provide an intervertebral implant which would employ a combination of non-sharp features to provide prompt fixation to promote osteointegration and to provide structural support primarily through the structurally strong cortical bone tissue of the vertebrae.